A typical projection system includes an arc-lamp source of bright white light, a method for separating the white light into red, green and blue spectral components, and a spatial light modulator (SLM, also called a light valve) for two dimensional imaging each of the spectral components to create a color picture. The SLM performs the spatial and temporal modulation in response to an analog or digital video signal generated by a computer or another video source such as a TV tuner, VCR, HDTV broadcast, or DVD player. The SLM typically creates sequential images in red, green, and blue color planes although other color separation techniques exist such as cyan, yellow, magenta, and optionally white planes. The color planes may then be recombined optically and projected onto a screen, or they may be flashed temporally in sequence onto the screen at such a rate that a viewer perceives only a single image.
Most digital projectors compete based on the number of screen lumens that the projector places on the projection screen. Although the competition in the projector market is fierce, cost considerations as well as size constraints have limited the development of more efficient optical designs for other light sources.
Most conventional small projector systems have total optical system efficiency from bulb to screen of less than 12%. That is, only up to 12% of the light created by the bulb actually exits from the projection optics and makes it to the screen. This inefficient design results in not only a dimmer display but also the consumption of needlessly wasted power. Several approaches have been attempted to increase the efficiency but with little success.
For instance, if an integrating rod is increased in size to gather more of the beam from the lamp focus then the magnification of the illumination system must be changed. Changing the magnification of the illumination system requires changes in optical path length of the system and the effective focal length of the lenses used in the system. These changes may or may not be practicable in products with small profiles or even in large profile digital projectors. Therefore, the need to capture more light from the lamp focus and couple it into the same size integrating rod with increased luminance is desired in all digital projectors.
In addition, there are several problems with existing arc-lamp sources. The most commonly used lamp source is a mercury vapor arc lamp. This lamp produces the most light for a given wattage and has a small point source. However, mercury arc lamps have a short lifetime compared to other technologies and produces light that is spectrally deficient in the red spectrum. In addition, mercury is a hazardous material that many countries would like limit the use of or ban outright. Although other bulb technologies could be substituted for the mercury vapor arc lamp, none has its efficiency and small spot size that allow for a large etendue and thus production of small high intensity projectors. Etendue is a method of measuring the system throughput. Etendue once created by a light source can only be increased. In a perfect lossless optical system, etendue is always conserved. Etendue in an optical system is limited by the element, which has the least etendue. An optical source system must have an etendue less than the limiting etendue or maximum system efficiency. Simply replacing the arc lamp bulb with a non-arc lamp will not offer a satisfactory competitive solution because of the non-point source nature of non-arc lamps limits their available etendue and they tend to produce emissions outside of visible light, which must be eliminated. To allow for substituting out mercury bulbs, the light path in a projector must have increased etendue efficiency and unwanted energy removal. Therefore, there is also a need for a solution that allows other bulb technologies to compete with mercury vapor arc lamps.
In summary, there exists a need to overcome the efficiency and other problems associated with arc lamp bulbs, particularly mercury vapor types.